Speed Control Flashcards
(25 cards)
what relationship does armature resistance affect
torque-speed
can armature current be increased or decreased
no it is an inherent feature of the machine
how to control the total speed
adding a resistor in series with RA, in series machines this reduces the field current too
drawback of adding R to control speed
additional loss due to additional R, thats why some motors get hot
how does RE(added R) affect the T and omega
for a given T, speed decreases as Re increases
what is soft starting of machine
starting motor gradually than suddenly, Re increases total R reducing starting current
a way to control speed
adjusting supply V across the aramture, change in vin = change in speed
How to adjust Vin
controllable power supply, V can be adjusted as need dc/dc converter, controlled rectifier
how does vin affect speed
for a given T, speed increases as Vin increases
what is 4 quadrant operation of a seperately excited motor
allows motor to operate in all directions and mode(motor, gen)
what are the 4 quadrants
spin forward
spin back
produce driving torque(motor)
resistive torque(gen)
speed and T is +
machine is motor since power is flowing into load
Vin > 0, Ia>0, speed>0
speed is + and T -
machine is generating since power is flowing from the load to supply
Vin<e, Ia<0, speed>0
motor resists motion slowing down load
speed is - and T +
reverse generating,
speed and T +
motor drives load in reverse
Vin<e, Ia<0, speed<0
what happens when you want to change motor speed from A to B meaning Vin changes to V3 where Vin>V3
drops from A to X (point in generating mode bc of intertia giveing energy back to supply) then decelerates to mototring at B
why is generating mode imortant even when it is not generator
gen mode occurs naturaly during decelration or speed reduction bc motor converts kinertic ene to elec eng
what is steady state
constant over time
transient
how characteristics change over time
dc machines have inductance in coil how does this affect Ia and speed
changes of current and speed isnt immediate because of machines inertia as there is change in machines developed torqu
formula for ia
ππ΄ = (1/π π΄) x (πππ β ππΈπ β πΏπ΄ (πππ΄/ππ‘))
speed change formula
π = (1/πΆπΉ) x (π β ππΏ β π½(ππ/ππ‘))
Cf = coefficient of viscous friction
J - moement of inrertia
dw/dt = angular acceleration
cf is considered small enough
TL = load torque
π½(ππ/ππ‘) = π β ππΏ what does this formula mean
to accelerate a machine, developed torqe has to be added
πΏt formula from sheet, what does it mean
time taken to change speed from speed1 to speed2
πΏπ‘ can be acceleration time if π2 > π1 or deceleration time if π2 < π1
